I'll say 1995, since that's the first year the World English Sporting Clays Championship was held in the USA. Contary to popular belief, it was not the NSCA who was primarily responsible for the change to a free mount.

American sporting shooters were actually determined that the new game of sporting clays would not be "dummied down" like American skeet and trap. Thus, the low gun start and a variable launch with up to a 3 second delay. Unfortunately, not only was the low gun position difficult to enforce, there were no real referees to enforce it; a situation that still exists. Good scorekeepers maybe, but darned few real referees who know and will enforce the rules. Which means most will call "no bird" when the target doesn't come out instantaneously because that's what pantywaist shooters have come to expect.

Anyway, at the end of 1994 the NSCA announced they were going to adopt the FITASC lowgun start for sporting clays. The Brits said "Wait a minute. The game you're playing is known as English Sporting. We're coming to the USA to shoot the English Sporting World Championship and we're going to shoot it by OUR rules....which means a free mount."

Down here in OZ, we have a similar situation, we have Field & Game rules where a line is on on a shooters coat/shirt/jacket 10" from top of shoulder. Butt of gun must be below that line and not mounted until target is in view. I believe that this is similar to FITASS rules.
Then we have the ACTA, who control our trap shooting, when they tried to have the gun down position adopted, all hell broke loose, so now gun mount is optional.Can't remember the last [ or the first] time I ever went huntin' with the gun pre-mounted.

And when Bubba and Cooter started releasing pigeons from "traps" out behind the barn I'm pretty sure they didn't have adjustable combs and release triggers.

Anybody wishing to simulate field hunting can do so with a low gun, safety on, and surprise pulls on a skeet field, trap field or sporting course. None of that bears any resemblance to clay target competition. No reason it should.